1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiple Access (OFDMA) system. More particularly, the present invention to a receiver and a reception method for estimating channels in an OFDMA system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a transmission scheme that converts a serial input data stream into N parallel data streams and carries the converted N parallel data streams on separate individual subcarriers, thereby increasing a data rate. When the OFDM transmission scheme is used for non-broadcast cellular mobile communication, wireless Local Area Network (LAN), wireless mobile Internet, etc., a Multiple Access scheme for multiple users is needed together with a single-carrier transmission scheme. Therefore, OFDMA is used as an OFDM-based Multiple Access scheme.
Orthogonal Frequency Division Multiple Access (OFDMA) is a scheme in which each user uses a number of subchannels and a number of OFDM symbols. In the OFDMA scheme, the user allocates the number of subcarriers and the number of OFDM symbols differently according to the transfer rate required by each user, thereby ensuring efficient resource distribution.
The terms ‘OFDM’ and ‘OFDMA’ will both be referred to herein as ‘OFDM’ unless stated otherwise.
In the OFDM system, high-speed data transmission is required. For the high-speed data transmission, high-order modulation schemes (e.g., 16-ary Quadrature Amplitude Modulation (16-QAM) and 64-ary QAM (64-QAM)) are needed. The high-order modulation scheme-based transmission method exerts an influence on performance according to the channel state. That is, the high-order modulation scheme has a very high transfer rate in a good channel state but requires retransmission in a poor channel state, thus experiencing greater performance degradation when compared with the low-order modulation schemes (e.g., Binary Phase Shift Keying (BPSK) and Quadrature PSK (QPSK)). Therefore, it is important to correctly estimate the channel state and use a modulation scheme suitable thereto.
A method for estimating the channel state in the OFDM system will be described. A transmitter transmits a base code or a pilot signal previously agreed upon with a receiver, and the receiver performs channel estimation using the base code or the pilot signal.
In the receiver of the OFDM system, a channel estimator greatly changes in performance according to frequency selectivity of the channel. The frequency selectivity is determined herein according to the time delay (or delay spread) characteristic of a multipath channel. That is, an increase in the time delay of a channel causes an increase in the frequency selectivity, and the increase in the frequency selectivity reduces channel estimation performance in the OFDM system using a pilot structure.
FIG. 1 is a block diagram illustrating a structure of a receiver in a conventional OFDM system.
Since the signal received at a receiver from a transmitter is an analog signal, an Analog-to-Digital Converter (ADC) 101 converts the received analog signal into a digital signal. An Automatic Frequency Control (AFC) unit 103 cancels a frequency offset through frequency control on the digital signal, and a Symbol Timing Recovery (STR) unit 105 sets an optimal Fast Fourier Transform (FFT) window for the frequency offset-canceled received signal. An FFT unit 107 performs an FFT on a received signal in the window to convert a time-domain received signal into a frequency-domain received signal, and a channel estimator 109 performs channel estimation by extracting a pilot signal from the frequency-domain received signal. An equalizer 111 performs channel equalization on the frequency-domain received signal using a channel impulse response estimated by the channel estimator 109. A Forward Error Correction (FEC) unit 113 extracts information bits by performing channel decoding on the channel-equalized input signal.
When the STR unit 105 in the conventional receiver of FIG. 1 sets an FFT window, Inter-Symbol Interference (ISI) may occur due to a precursor of a channel.
FIG. 2 is a diagram illustrating an example in which ISI occurs during an FFT window setting in the conventional STR unit of FIG. 1.
In the receiver, a received signal is expressed as a sum of several transmission signals 202 as shown in Equation (1), due to time delays 201 of channels.
                                                                        y                ⁡                                  (                  n                  )                                            =                            ⁢                                                h                  ⁡                                      (                    n                    )                                                  *                                  x                  ⁡                                      (                    n                    )                                                                                                                          =                            ⁢                                                (                                                            ∑                                              l                        =                        0                                                                    L                        -                        1                                                              ⁢                                                                  h                        l                                            ⁢                                              δ                        ⁡                                                  (                                                      n                            -                                                          τ                              l                                                                                )                                                                                                      )                                *                                  x                  ⁡                                      (                    n                    )                                                                                                                          =                            ⁢                                                ∑                                      l                    =                    0                                                        L                    -                    1                                                  ⁢                                                      h                    l                                    ⁢                                      x                    ⁡                                          (                                              n                        -                                                  τ                          l                                                                    )                                                                                                                              (        1        )            
In Equation (1), h(n) denotes a channel impulse response, x(n) denotes a transmission signal corresponding to one OFDM symbol, y(n) denotes a received signal corresponding to one OFDM symbol, and n denotes a discrete time index. Further, in Equation (1), L denotes the number of multipaths, l denotes an index of a multipath, hl denotes a channel impulse response of each multipath, and τl denotes a time delay of each multipath.
To prevent ISI, the OFDM system inserts a Guard Interval (GI) 203 into an OFDM symbol. In this case, the GI 203 generally has a longer length than the maximum time delay of the channel in the time domain.
With reference to FIG. 2 and under the assumption that the multipath channel is composed of two multipaths, a description will be made of an example in which ISI occurs due to a channel precursor.
Referring to FIG. 2, the STR unit 105 sets the part obtained by excepting a GI from a received signal, as an FFT window 204 of an Nth OFDM symbol, on the basis of the path having the highest power among the multipaths. However, when a precursor exists in the channel, a part 205 of an (N+1)th OFDM symbol is included therein by the channel precursor in the time-domain signal where the FFT window 204 of an Nth OFDM symbol is set, thereby generating ISI. In order to address this problem, the STR unit 105 time-advances, as shown by reference numeral 206, an FFT window for an Nth OFDM symbol on the basis of the multipath that first occurred in the time domain among the multipaths, in consideration of a non-estimated additional channel precursor.
A received signal in an FFT window having N samples is defined as Equation (2).
                                                                        X                ⁡                                  (                  k                  )                                            =                            ⁢                                                FFT                  N                                ⁢                                  {                                      x                    ⁡                                          (                      n                      )                                                        }                                                                                                                        =                                ⁢                                                      ∑                                          n                      =                      0                                                              N                      -                      1                                                        ⁢                                                            x                      ⁡                                              (                        n                        )                                                              ·                                          exp                      ⁡                                              (                                                                              -                            j                                                    ⁢                                                                                    2                              ⁢                              π                              ⁢                                                                                                                          ⁢                              kn                                                        N                                                                          )                                                                                                        ,                              k                =                0                            ,              1              ,              …              ⁢                                                          ,                              N                -                1                                                                        (        2        )            
If the FFT window is time-advanced by m samples to prevent ISI, x(n) undergoes circular rotation by m samples due to GI. An OFDM symbol that underwent m-sample circular rotation is defined as Equation (3) after undergoing an FFT.
                                                                                                              X                    ~                                    m                                ⁡                                  (                  k                  )                                            =                            ⁢                                                FFT                  N                                ⁢                                  {                                                            x                      N                                        ⁡                                          (                                              n                        -                        m                                            )                                                        }                                                                                                                        =                                ⁢                                                      X                    ⁡                                          (                      k                      )                                                        ·                                      exp                    ⁡                                          (                                                                        -                          j                                                ⁢                                                                              2                            ⁢                            π                            ⁢                                                                                                                  ⁢                            k                            ⁢                                                                                                                  ⁢                            m                                                    N                                                                    )                                                                                  ,                              k                =                0                            ,              1              ,              …              ⁢                                                          ,                              N                -                1                                                                        (        3        )            
In Equation (3), xN(n) means x(n), to which circular rotation using N as a modulus is applied. That is, xN(n) is expressed as Equation (4).xN(n)=x(n mod N), n=0,1, . . . , N−1  (4)
When the FFT window is shifted by m samples along the time domain as shown in Equation (3), additional phase rotation occurs in the pre-shifting frequency response as shown in FIGS. 3 and 4.
FIGS. 3 and 4 are diagrams illustrating a channel impulse response and a frequency response for m samples in a conventional OFDM system.
Referring to FIG. 3, a channel impulse response and a frequency response are illustrated for m=0, i.e. when an FFT window is set on the basis of the first multipath. Referring to FIG. 4 shows a channel impulse response and a frequency response are illustrated for m=4, i.e. when an FFT window is set 4 samples in advance of that for the first multipath. In FIGS. 3 and 4, making a comparison between a frequency response for m=0 and a frequency response for m=4, the frequency response for m=4 is higher in frequency selectivity since it is higher in a change rate than the frequency response for m=0.
In the conventional OFDM system, the receiver performs channel estimation with a method using pilot signal-based linear interpolation in order to reduce complexity. As a result, the frequency selectivity of channels has a direct influence on the channel estimation performance. That is, as in the examples of FIGS. 3 and 4, it is advantageous to set an FFT window after sufficiently shifting it forward while taking the channel precursor into account, in order to prevent ISI. However, in this case, channel estimation performance may be reduced in the situation where no ISI occurs.
Therefore, there is a demand for a receiver and a reception method for solving the channel estimation performance reduction problem caused by the FFT window setting at the receiver of the conventional OFDM system, and for improving channel estimation performance of a channel having a large time delay.